1. Field of the Invention.
The present invention relates generally to successive approximation analog-to-digital converters. In particular, the present invention is a single chip successive approximation analog-to-digital converter with a trimmable and controllable digital-to-analog converter.
2. Description of the Prior Art.
Successive approximation analog-to-digital converters (ADCs) are in widespread use and are disclosed, for example, in the following United States patents:
______________________________________ Inventor U.S. Pat. No. ______________________________________ Brokaw et al. 4,400,689 Brokaw et al. 4,427,973 Shibata et al. 4,441,198 Brokaw et al. 4,547,766 Brokaw et al. 4,556,870 Collings 4,571,507 ______________________________________
Analog-to-digital converters of this type include a digital-to-analog converter (DAC) with its inputs fed from a successive approximation register (SAR). To begin a conversion, the DAC inputs are first set to an initial value (usually half-scale), and a comparator determines whether the input signal voltage is greater than or less than the DAC output voltage. This partial result and those subsequently determined partial results are held by the SAR. The SAR also sets up the DAC input values for succeeding comparisons as a function of previous comparisons in accordance with a successive approximation algorithm. This process is repeated until all bit values are determined.
Successive approximation analog-to-digital converters having twelve or more bits usually require some type of post-fabrication trimming (e.g., laser trimming) to achieve necessary accuracy. This is due to the fact that the "natural" matching of the integrated components is insufficient. It is the digital-to-analog converter which must be trimmed, since overall accuracy (linearity) of the conversion is set by its accuracy.
Most currently available twelve-bit ADCs of the successive approximation type have been made with hybrid techniques, and use laser trimming. In this approach, the DAC is fabricated on a separate chip from other components, thereby permitting separate trimming to required accuracy with full access to the DAC inputs. For a one-chip monolithic successive approximation ADC, however, this is not possible. The DAC inputs are "buried" on the chip and are not externally controllable.
It is evident that there is a continuing need for improved analog-to-digital converters of the successive approximation type. The ADC must be fabricated as a monolithic one-chip device in order to be produced at commercially viable prices. External control of the DAC inputs is required to permit trimming to required specifications. Power consumption and the number of transistors required for implementation must, of course, be kept to a minimum.